Issue is, because of scsi0:1.writeThrough = "true" in vmx file for 2 HDD, whenever VM migrated to ESXi 5.1 2nd HDD disappear within OS and we have to migrate it back to ESXi 4.1 to resolve the issue. after investigation we found that those VMs were deployed using the BMC orchestrator automation and 2 HDD was added at the time of VM deployment. Now if we remove and re-add the 2nd HDD above entry goes away from vmx file and then VM works fine on ESXi 5.1.
Now Issue is I need to find out the VMs those have scsi0:1.writeThrough = "true" entry in VMX file so that we can remove this entry from vmx file and upgrade the HOSTS to ESXi 5.1. Our upgrade project is stuck because of this issue.